A long-standing problem in the steel making industry has been the ability to control or minimize the carryover of slag during the tapping of a BOF converter. Tapping is the pouring of molten metal from a BOF converter into a corresponding ladle, with the metal flowing from the converter through a taphole defined therein.
During the manufacture of steel, molten iron (known as hot metal) having impurities (e.g. C, Si, Mn, S, P, etc.) therein is typically introduced into a converter vessel known as a basic oxygen furnace (BOF). In the BOF converter, gaseous oxygen (O.sub.2) is injected or jetted onto the hot metal in order to remove the impurities to desirable levels. During this purification process, fluxes such as lime (Cao) and MgO are added into the furnace and combine with oxides such as SiO.sub.2, MnO, and FeO formed during the oxidation process to form molten "slag" in the converter. This slag floats on top of the molten steel in the BOF converter, because the slag's density is less than that of the molten steel.
After the oxygen is introduced into the BOF converter for an extended period of time (e.g. from about 16-25 minutes depending upon the volume of the BOF converter, the amount of molten iron therein, and the grade of the steel to be made) and the molten slag and steel have formed, the converter vessel is tilted and tapped. During tapping, molten steel is poured from a taphole in the side of the BOF converter into a ladle located below same. It is during this tapping that undesirable slag carryover can occur.
When the BOF converter vessel is properly tapped, a small amount of carryover may occur at the beginning of tapping, but the slag carryover of most concern occurs at the end of tapping when most of the substantially purified molten steel has already been poured into the ladle below, and mostly slag (instead of mostly steel) remains in the BOF converter. When a typical BOF converter is tilted to a pouring position for tapping, the molten steel is poured from the taphole located in the side of the converter before the slag is poured, due to the different densities of the two molten materials. If the operator(s) tapping the converter does not stop tapping (or pouring) at about the precise instant when the molten slag begins to flow through the taphole, the undesirable molten slag is also poured into the ladle below on top of the already poured molten steel. When too much slag is poured into the ladle from the BOF converter, this affects the cleanliness and reintroduces impurities such as phosphorus (P) into the steel, adversely affects the aluminum efficiency during tap, and prevents certain grades of steel from being made. Any attempt to remove or minimize the effect of excess slag poured into the ladle is expensive, time-consuming, and/or labor intensive. For example, if too much slag is accidentally poured into the ladle, hundreds of dollars worth of alumina or other slag modifier(s) may have to be added to the molten ladle slag to try to minimize the levels of FeO and other unstable oxides in the slag. In sum, minimizing slag carryover from the BOF converter into the ladle is essential for efficient manufacturing of high quality steel.
Many techniques have been used in an effort to control the carryover of slag during the tapping of BOF converters. For example, see Slag Carryover in Oxygen Converters: an International Review, by Da Silva, Bergman, and Lindfors [pp. 91-95], the disclosure of which is hereby incorporated herein by reference. In this review, numerous methods for controlling the carryover of slag during BOF converter tapping are discussed. For example, it is known to use refractory plugs, metallic plugs, wooden plugs, fiber plugs, gunned clay, dart-shaped floating elements, and ball-shaped floating elements in an attempt to control or minimize slag carryover.
Certain known techniques result in the interruption of the metal pour or tap stream from the converter near the end of tapping in order to minimize slag carryover. Dart-shaped and ball-shaped floating elements are often used for this purpose. In FIGS. 4 and 5 of the above-referenced article, the often unsatisfactory results associated with these conventional methods are illustrated. For example, dart- and ball-shaped floating elements are known to be unsuccessful when the slag is thick or viscous, and it has been found that the positioning of these floating elements inside the converter is both difficult and critical. The structure of the taphole also affects the effectiveness of these types of floating elements. As discussed in the article, some steel plants have reported that the balls sometimes close the taphole too early, which results in the leaving of purified molten steel (affecting yield) in the converter or extending the tap time (this is both expensive and inefficient). Accordingly, it is known in the art that while floating elements may help to minimize slag carryover, they are often inefficient and the results are unpredictable. Still further, both balls and darts are undesirably expensive.
Despite the fact that so many slag carryover prevention techniques are known, it is stated at the conclusion of the above-referenced article that "none of the methods in use today can be considered to be of universal application, since each has its limitations and can only reach the expected results if specific conditions exists." In other words, there has existed a longstanding need in the art for a system and corresponding method for minimizing the carryover of slag during the tapping of BOF converters, which is usable in different environments by operators of different skill levels. No known technique has, to date, been found to be satisfactory in all commercial steel-making environments because many techniques are not considered to be efficient enough and others are too expensive for use with ordinary steel grades.
In view of the inefficiency and non-effectiveness of known BOF slag carryover prevention methods, many steel plants simply rely upon operators to visually detect when the slag portion of tapping is reached. Unfortunately, this method of slag carryover prevention is inefficient at best, as it is nearly impossible for most humans to visually observe any visible difference between purified molten steel being poured from the converter taphole and molten slag being poured from the taphole [both are molten and yellow to white-hot].
U.S. Pat. No. 4,222,506 discloses another method for controlling slag carryover, primarily directed toward detecting slag in a pour from a ladle into a tundish. In the '506 patent, an infrared (IR) camera looks at the molten metal being poured and attempts to detect the presence of slag therein. Unfortunately, it has been found that the system disclosed in the '506 patent is inefficient and is incapable of easily, satisfactorily, and consistently detecting BOF-to-ladle slag in steel mill environments. In column 2 of the '506 patent, an emissivity value of about 0.28 is mentioned. As can be seen below, this emissivity value may indicate that short IR wavelengths in the short-IR region (i.e. at approximately 3-5 .mu.m) are being measured by the camera in the '506 patent in order to detect slag. However, it is important to note that the '506 patent does not recognize or appreciate any importance to any potential wavelength. It has been found that utilizing short IR wavelengths in this range is insufficient to efficiently detect the tapping of slag in BOF-to-ladle steel mill environments. Accordingly, the '506 patent suffers from at least the following problems.
First, the environment within which a BOF converter is tapped so as to allow molten steel to flow into a ladle often includes much more airborne dust, smoke, gases, and particulate matter than ladle-to-tundish environments. During BOF converter tapping, much smoke, gas, and particulate matter is typically emitted into the air surrounding the tap stream. This, at times, makes it very difficult for humans to view the tap stream through the smoke, gases, and other airborne particulate matter. Airborne particulate matter can block radiation with wavelengths smaller than the size of the particle(s). In the BOF environment, the size of the particles is such that the long wavelengths of radiation are most likely able to reach an IR camera. Smaller wavelengths on the other hand will often be blocked. Because short (near) IR wavelengths, including those in the range indicated by the '506 patent, are susceptible to being blocked by dust and other airborne particulate matter, the system of the '506 patent at times is unable to accurately differentiate between slag and steel during tapping.
Second, the gases emitted during BOF-to-ladle tapping also have an adverse effect upon the IR wavelengths indicated by the '506 system. As will be discussed herein, gases such as CO.sub.2 and H.sub.2 O emitted proximate the tap stream in BOF environments absorb certain IR wavelengths, notably those in the 3-5 .mu.m range (e.g. at about 4.2 um). Thus, in BOF environments, the wavelengths used in the '506 patent often cannot be seen by the IR camera. This is yet another problem, because, without seeing these wavelengths, the image is unclear and the '506 camera may not be able to differentiate between slag and steel during tapping.
A third problem with the slag detection system of the '506 patent is that it appears to be positioned rather close to the molten tap stream being poured from the ladle into the tundish. It has been found by the instant inventors that the positioning of an IR camera in close proximity to a tap stream sometimes results in less than satisfactory tap stream slag readings due to the high temperature background surrounding the tap stream in BOF environments. When aiming the camera at the mouth of the ladle, the background is undesirably bright slag in the ladle, and this tends to affect the clarity of the image. Because of the appearance of a "hot" background, it may be difficult to differentiate between slag and steel in the '506 patent.
In view of the above, it is clear that the system of the '506 patent is less than desirable in BOF environments for many reasons. This is believed to be a result of the '506 patent being primarily designed for detecting the presence of slag in a ladle-to-tundish environment, as opposed to a BOF-to-ladle environment where many more gases and other airborne particulate matter are present. It is noted, however, that the '506 patent does discuss and illustrate that it is also possible to use it in the work of a converter into a ladle.
Still another approach used by many in the trade to minimize slag carryover in BOF environments is the positioning of electromagnetic coils on BOF converter tapholes. By monitoring such a coil(s), it is possible to determine when slag is beginning to flow through the corresponding taphole. Upon the coil detecting slag, the taphole may be closed or the converter may be tipped upward to stop tapping. Unfortunately, electromagnetic coils are problematic in that they are positioned within the converter, and often break down or fail rather frequently. Another problem with coils is that they produce only an alarm, while the melter (i.e. operator) is still looking at the tap stream to make sure that slag is being poured before stopping tap. With slag splashing, converters operate for months and months at a time through many heats (e.g. up to about 20,000 heats or for up to one and one half years). Thus, if the coil in the taphole fails, there is no way to replace or perform maintenance on it without stopping BOF operation. In all practicality, there can be no new coil until the next BOF refractory relining. This is highly undesirable, reduces yields, cuts down on efficiency, and quickly becomes rather expensive.
It is apparent from the above that there exists a need in the art for a system and method for minimizing the carryover of slag during the tapping of a BOF converter in the manufacture of steel, wherein the system/method improves reliability relative to prior art techniques, has a higher success rate than prior art techniques, results in improved slag detection, and reduces maintenance costs relative to known techniques.
It is a purpose of this invention to fulfill the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.